This invention relates to a method and apparatus for applying a uniform electrical charge to a structure, and more particularly to a method and apparatus for applying a uniform corona produced charge to an electrophotographic member or imaging surface.
It is well known that in electrostatic printing equipment a corona generating device including a corona discharge electrode is employed to place positive or negative charges onto a photoconductive member or surface. The photoconductive member or surface is then exposed to a pattern of light which corresponds to the image to be printed. The pattern of light will discharge the photoconductive surface selectively in accordance with the presence and intensity of the light creating an electrostatic image of the light pattern on the surface. This electrostatic image may be employed in a number of ways now well known in the art in order to reproduce an image on a sheet of paper, or in some instances, the surface or member may be fixed in order to reproduce the electrostatic image.
The nature of photoconductive member is such that it will retain the charge deposited thereon for a very short time period, and only then if maintained in a darkened environment. If it takes some period of time for the charge to be applied to the member the level or intensity of the charging process will have decayed or reduced as compared to the charge level applied at the end of the charging process. If the lighted image is exposed after the entire charging process the electrostatic image produced may be nonuniform as a result of this variation in charge level.
The corona generated in the above noted devices could be positively or negatively biased either in order to produce a positive or negative charge depending upon the nature of the photoconductive surface employed. When a positive corona is generated from a metallic filament electrode, the resultant charge applied to the photoconductive surface is generally relatively uniform due to the uniformity of the positive corona electrode emission. Many of the more currently available devices require a negative corona. When a negative corona is generated from a metallic filament electrode, the photoconductive surface obtains a charge which varies in density from point to point due to the nonuniform negative corona electrode emission. It is believed that this nonuniformity in charge is manifest in the developed image since areas containing a higher charge will attract more electrostatic developer material thereto thereby creating a streaked image appearance.
A number of devices have been developed in order to provide a uniform charge on the desired photoconductive surface. One such device employs specially coated electrodes which suppress the widely spaced emission nodes common to negatively biased corona electrode emissions. Another device moves the metallic corona electrode and the surface being charged substantially in orthogonal directions. Still other devices employ alternating currents plus a high voltage direct current to minimize or reduce the nonuniformity. These devices appear to provide a more uniform charge for the above equipment. It should be noted, however, that the above mentioned equipment generally is rather limited in its photographic reproduction capabilities to reproducing printed matter, because of the nature of the photoconductive surfaces employed.
Electrophotographic members are being developed which are much more sensitive than the members employed in the above noted equipment. These electrophotographic members are of a quality capable of reproducing or creating high resolution images, that is, each point on the surface of the member is capable of selectively discharging in accordance with the intensity of incident light so that an almost infinite scale of grey tones can be reproduced in the resultant image. In order to make full use of this feature, the applied corona charge must be substantially uniform across the entire member or surface of the member. This is necessary in order to produce a resultant image which has varying shades that result from variations in the intensity of incident light and not from variations in the initial corona produced charge.